Chalcogenide Nanocrystals Research Articles

Effect of Pressure on Interband and Intraband Transition of Mercury Chalcogenide Quantum Dots

Mercury chalcogenide nanocrystals generate a lot of interest as active materials for low cost infrared sensing. Device improvement requires building a deeper understanding of their electronic structure which combines inverted band ordering, quantum confinement and dependence to surface chemistry. This is particularly true with the development of mercury chalcogenide colloidal heterostructures (HgSe/HgTe, HgTe/CdS…). In this case the lattice mismatch induces a strain which affects significantly the band gap given the narrow band gap nature of the material. Here we study the effect of pressure on interband and intraband transitions in a series of HgTe and HgSe colloidal quantum dots. We demonstrate that in HgTe and HgSe, the nanocrystal morphology stabilizes the zinc blende phase up to 3 GPa. Under compressive strain, the interband signal blueshifts by 60 meV/GPa, while the intraband transition redshifts by a small amount (8 meV/GPa). Using an 8-band k·p formalism, we reveal that the interband shift has the same origin as the one observed for bulk material (change of effective mass, followed by band gap opening), while the intraband shift can be attributed to an increase of effective mass only.

Doping Mn(II) in All-Inorganic Ruddlesden-Popper Phase of Tetragonal Cs2PbCl2I2 Perovskite Nanoplatelets.

DopingMn(II) in inorganic Ruddlesden-Popper phase Cs2PbCl2I2 perovskite nanoplatelets is reported. The host nanostructures were prepared with a calculative protocol taking the exact required composition of Cs(I) and Pb(II) and injectingthepreformed mixed oleylammonium chlorides and iodides at optimized reaction temperature. Reactions were optimized with various halides and their mixtures, but the stable phase of the Cs2PbX4 system was obtained only for the chloride-iodide mixed-halide system. Introduction of Mn(II) along with Pb(II), resulted in successful light-emitting doped nanocrystals. Measuring the photoluminescence and the decay lifetimes at room and liquid nitrogen temperatures, the variations in the excitonic, self-trapped, and Mn dopant emission properties were compared with those of the chalcogenide and perovskite nanocrystals.

Dye-Sensitized Ternary Copper Chalcogenide Nanocrystals: Optoelectronic Properties, Air Stability, and Photosensitivity

We report on the effect of ligand exchange of copper sulphide selenide as well as copper selenide nanocrystals with the organic pi-system Cobalt beta-tetraaminophthalocyanine and analyse changes in the structural, optical as well as electric properties of thin films of these hybrid materials. A strong ligand interaction with the surface of the nanocrystals is revealed by UV-vis absorption and Raman spectroscopy. Grazing-incidence small-angle X-ray scattering studies show a significant contraction in the interparticle distance upon ligand exchange. For copper-deficient copper selenide, this contraction has a negligible effect on electric transport, while for copper-deficient copper sulphide selenide, the conductivity increases by eight orders of magnitude and results in metal-like temperature-dependent transport. We discuss these differences in the light of varying contributions of electronic vs. ionic transport in the two materials and highlight their effect on the stability of the transport properties under ambient conditions. With photocurrent measurements, we demonstrate high optical responsivities of 200-400 A W-1 for the phthalocyanine-capped copper sulphide selenide and emphasize the beneficial role of the organic pi-system in this respect, which acts as an electronic linker and an optical sensitizer at the same time.

Structure-Selective Synthesis of Wurtzite and Zincblende ZnS, CdS, and CuInS2 Using Nanoparticle Cation Exchange Reactions.

For polymorphic solid-state systems containing multiple distinct crystal structures of the same composition, identifying rational pathways to selectively target one particular structure is an important synthetic capability. Cation exchange reactions can transform a growing library of metal chalcogenide nanocrystals into different phases by replacing the cation sublattice, often while retaining morphology and crystal structure. However, only a few examples have been demonstrated where multiple distinct phases in a polymorphic system could be selectively accessed using nanocrystal cation exchange reactions. Here, we show that roxbyite (hexagonal) and digenite (cubic) Cu2- xS nanoparticles transform upon cation exchange with Cd2+, Zn2+, and In3+ to wurtzite (hexagonal) and zincblende (cubic) CdS, ZnS, and CuInS2, respectively. These products retain the anion and cation sublattice features programmed into the copper sulfide template, and each phase forms to the exclusion of other known crystal structures. These results significantly expand the scope of structure-selective cation exchange reactions in polymorphic systems.

Colloidal CdSe Nanoplatelets, A Model for Surface Chemistry/Optoelectronic Property Relations in Semiconductor Nanocrystals

While the surface termination of quasi-spherical metal chalcogenide nanocrystals or quantum dots has been widely investigated, it remains unclear whether the ensuing surface chemistry models apply to similar nanocrystals with anisotropic shapes. In this work, we report on the surface-chemistry of 2D CdSe nanoplatelets, where we make use of an improved synthesis strategy that yields stable and aggregation free nanoplatelet suspensions with a photoluminescence quantum yield as high as 55%. We confirm that such nanoplatelets are enriched in Cd and, by means of 1H nuclear magnetic resonance spectroscopy, we show that the Cd-rich surface is terminated by X-type carboxylate ligands. Not unlike CdSe quantum dots (QDs), entire cadmium carboxylate entities can be displaced by the addition of amines, and the desorption isotherm points toward a considerable binding site heterogeneity. Moreover, we find that even the slightest displacement of cadmium carboxylate ligands quenches the nanoplatelet photoluminescence. These experimental findings are further confirmed by density functional theory (DFT) calculations on a 5 monolayer model CdSe nanoplatelet. These simulations show that the most labile ligands are located in the vicinity of facet edges, and that the displacement of ligands from such edge sites creates midgap states that can account for the observed photoluminescence quenching. Next to extending surface chemistry insights from colloidal QDs to nanoplatelets, this work indicates that CdSe nanoplatelets constitute a unique nanocrystal model system to establish a comprehensive description of midgap trap states, which includes their structural, chemical, and electronic properties.

Laser operation by kesterite chalcogenide nanocrystals

We demonstrate a possibility to vary structural, morphological and optoelectronics parameters for kesterite Cu2ZnSnS4 nanocrystalline films using external laser beams at 1540 nm, and 770 nm or 1064 nm, and 532 nm. The illumination is performed both by single mode laser beams as well as by bicolor coherent treatment. The principal changes were observed for the density of electronic states in the valence band, third-order nonlinear optics, surface states. The energy density of the fundamental laser beams was continuously varied up to 250 J/m2. The role of photo-thermal and phonon anharmonic contributions play here crucial role. The maximal changes of the electronic density of states and effective masses were achieved during simultaneous illumination by 1540 nm/770 nm Er: glass laser coherent bicolor laser beams. This result correlates well with the occurrence of the optical interference gratings, the formation of the nanocrystallites and surface morphology.

Effects of Electric Fields on Multiple Exciton Generation.

Unique properties of lead chalcogenides have enabled multiple exciton generation (MEG) in their nanocrystals that can be beneficial in enhancing the efficiency of third-generation solar cells. Although the intrinsic electric field plays an imperative role in a solar cell, its effect on the multiple exciton generation (MEG) has been overlooked, so far. Using EOM-CCSD as a many-body approach, we show that any electric field can affect the absorptivity spectra of the lead chalcogenide nanocrystals (Pb4 Te4 , Pb4 Se4 , and Pb4 S4 ). The same electric field, however, has insignificant effects on the MEG quantum probabilities and the thresholds in these nanocrystals. Furthermore, simulations show that Pb4 Te4 , among the aforementioned nanocrystals, has the lowest MEG threshold and the strongest absorptivity peak that is located in the multi-excitation window, irrespective of the field strength, making it the most suitable candidate for MEG applications. Simulations also demonstrate that an electric field affects the MEG characteristics in the Pb4 Te4 nanocrystal, in general, less than it perturbs MEG characteristics in Pb4 Se4 and Pb4 S4 nanocrystals. Our results can have a great impact in designing optoelectronic devices whose performance can be significantly influenced by MEG.

Phase-Controlled Synthesis of High-Bi-Ratio Ternary Sulfide Nanocrystals of Cu1.57 Bi4.57 S8 and Cu2.93 Bi4.89 S9.

High Bi-ratio ternary sulfides have been recently reported as superior thermoelectric materials. However, the synthesis of high Bi-ratio Cu-Bi-S nanocrystal remains a challenge. Reported here are the synthesis and characterization of three-phase Cu-Bi-S nanocrystals with the nominal chemical formulae of Cu1.57 Bi4.57 S8 , Cu2.93 Bi4.89 S9 and Cu3 BiS3 . The samples were prepared using a Bi2 S3 precursor by varying the amount and type of Cu2-x S (i. e. Cu2 S or Cu7.2 S4 ) reactants. TEM images reveal that two new samples crystalized having nanorod morphology with radii of approximately 50 nm and lengths of 200 nm. XPS results indicate that the valence states of Bi in both the two new phases are +3 with viable oxidation states for Cu. UV-Vis-NIR absorption spectroscopy reveals that narrow direct bandgaps are 1.12 and 1.27 eV for Cu1.57 Bi4.57 S8 and Cu2.93 Bi4.89 S9 , respectively. Besides, this method could also be applied to synthesize the Cu3 BiS3 phase with a new nanoplate morphology. The as-synthesized Cu-Bi-S samples show Cu/Bi ratio-dependent photoresponsive properties. This study not only reports the structure and bandgap of two ternary sulfides, which have only been discovered in the mineral previously, but also provides an efficient method for synthesizing Bi-rich ternary chalcogenide nanocrystals.

Utilizing Diselenide Precursors toward Rationally Controlled Synthesis of Metastable CuInSe2 Nanocrystals

Within the past decade, there has been an emergence of reports regarding the synthetic isolation of multinary metal chalcogenide nanocrystals that persist under ambient conditions with metastable c...

Dot–Wire–Platelet–Cube: Step Growth and Structural Transformations in CsPbBr3 Perovskite Nanocrystals

While the classical mechanism for the growth of colloidal chalcogenide nanocrystals is largely understood, fundamental insights for the growth of perovskite nanocrystals still remain elusive. Using nanoclusters of ∼0.6 nm diameter as monomers and growing to more than 25 nm in a single reaction, herein, the step growth process of perovskite CsPbBr3 nanocrystals is reported. This is performed with a step-rise of the reaction temperature with correlating annealing time. The growth is so precise that ∼0.6 nm (nearly one unit cell) increments were successively monitored in parallel with the conversion of clusters to nanowires and then to thickness tunable platelets and finally to size-tunable cube-shaped nanostructures. The entire reaction was monitored optically and microscopically, and their step growths were correlated. From these observations, the possible growth mechanism for perovskite nanocrystals along with their shape transformations was proposed.

Controllable Colloidal Synthesis of Tin(II) Chalcogenide Nanocrystals and Their Solution-Processed Flexible Thermoelectric Thin Films.

A systematic colloidal synthesis approach to prepare tin(II, IV) chalcogenide nanocrystals with controllable valence and morphology is reported, and the preparation of solution-processed nanostructured thermoelectric thin films from them is then demonstrated. Triangular SnS nanoplates with a recently-reported π-cubic structure, SnSe with various shapes (nanostars and both rectangular and hexagonal nanoplates), SnTe nanorods, and previously reported Sn(IV) chalcogenides, are obtained using different combinations of solvents and ligands with an Sn4+ precursor. These unique nanostructures and the lattice defects associated with their Sn-rich composition allow the production of flexible thin films with competitive thermoelectric performance, exhibiting room temperature Seebeck coefficients of 115, 81, and 153 μV K-1 for SnS, SnSe, and SnTe films, respectively. Interestingly, a p-type to n-type transition is observed in SnS and SnSe due to partial anion loss during post-synthesis annealing at 500 °C. A maximum figure of merit (ZT) value of 0.183 is achieved for an SnTe thin film at 500 K, exceeding ZT values from previous reports on SnTe at this temperature. Thus, a general strategy to prepare tin(II) chalcogenide nanocrystals is provided, and their potential for use in high-performance flexible thin film thermoelectric generators is demonstrated.

Exciton dissociation dynamics and light-driven H2 generation in colloidal 2D cadmium chalcogenide nanoplatelet heterostructures

Solar-to-H2 conversion is attracting much research attention as a potential approach to meet global renewable energy demands. Although significant advances have been made using metal-tipped colloidal cadmium chalcogenide zero-dimensional (0D) quantum dots and one-dimensional (1D) nanorod heterostructures in solar-to-H2 conversion, their efficiency may be further enhanced using an emerging class of colloidal cadmium chalcogenide nanocrystals, namely two-dimensional (2D) nanoplatelets (NPLs), because of their unique properties. In this review, we summarize the recent advances on exciton dissociation dynamics and light-driven H2 generation performance of colloidal nanoplatelet heterostructures. Following an introduction on the electronic structure of 2D NPLs, we discuss the dynamics of exciton dissociation by electron transfer to molecular acceptors. The exciton quenching dynamics of CdS NPL-Pt and CdSe NPL-Pt heterostructures are compared to highlight the effect of material properties on the relative contributions of the energy-transfer and electron-transfer pathways. Representative solar-to-H2 conversion performances of 2D NPL-metal heterostructures are discussed and compared with those of 1D nanorod-metal heterostructures. Finally, we discuss the challenges in further improving the solar-to-fuel conversion efficiencies of these systems.

Exploiting Crystallographic Regioselectivity To Engineer Asymmetric Three-Component Colloidal Nanoparticle Isomers Using Partial Cation Exchange Reactions.

The precise placement of different materials in specific regions of a nanocrystal is important for many applications, but this remains difficult to achieve synthetically. Here we show that regioselectivity during partial cation exchange reactions of metal chalcogenide nanocrystals emerges from crystallographic relationships between the precursor and product phases. By maximizing the formation of low-strain interfaces, it is possible to rationally integrate three distinct materials within uniform spherical and rod-shaped colloidal nanoparticles to produce complex asymmetric heterostructured isomers. Through sequential partial exchange of Cu+ in Cu1.8S nanocrystals with Zn2+ and Cd2+, five distinct ZnS/CdS/Cu1.8S nanosphere and nanorod isomers are accessible.

Oxidative Addition to SnII Guanidinate Complexes: Precursors to Tin(II) Chalcogenide Nanocrystals

SnS, SnSe and SnTe are potentially important semiconductor materials. Here, we describe the application of chalcogen containing SnIV guanidinate precursors for the production of tin(II) chalcogenide nanocrystals. Reaction of the stannylene(II) guanidinate complex [{Me2NC(NCy)2}2Sn] (1) with Ph2E2 (E = S, Se, Te), and CBr4 forms the SnIV complexes [{Me2NC(NCy)2}2Sn(Ch‐Ph)2] (2–4) and [{Me2NC(NCy)2}2SnBr2] (5), respectively. Complex 5 has been subsequently used for the synthesis of the corresponding SnIV mono chalcogenide complexes, [{Me2NC(NCy)2}2Sn = E] (6–8) by the reaction of 5 with Li2E systems. Isolated tin complexes have characterized by elemental analysis, NMR spectroscopy, and the molecular structures of complexes 2–5 determined by single‐crystal X‐ray diffraction. TG analysis showed that complexes 2–4 and 6–8 all have residual masses close to those expected for the formation of the corresponding “SnE” systems. Complexes 6–8 were assessed for their utility in the formation of nanocrystalline materials. The materials obtained were characterized by powder X‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM) and energy dispersive X‐ray analysis (EDX). Analysis showed formation of SnSe and SnTe from complexes 7 and 8, respectively.

Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se Nanocrystals

Steady-state intraband transition, which is a promising electronic transition of a colloidal quantum dot along with the band-gap transition, had been a long-standing challenge. The steady-state intraband transition occurring between discrete electronic states in the conduction band of a colloidal nanocrystal has been reported only from mercury chalcogenide nanocrystals for the past few years. Concerns about the toxicity of the mercury compound necessitate a new nontoxic system exhibiting a steady-state intraband transition. Here we present the steady-state intraband absorption and photoluminescence of Ag2Se colloidal nanocrystals under ambient conditions. The mid-IR intraband transition is carefully investigated by means of FT-IR emission spectroscopy, spectroelectrochemistry, compositional analysis, and transfer characteristics. Especially, the mid-IR intraband photoluminescence of the Ag2Se colloidal nanocrystal will open new avenues in the use of quantum-confined colloidal systems for mid- and long-wav...

Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals.

Mercury chalcogenide nanocrystals and especially HgTe appear as an interesting platform for the design of low cost mid-infrared (mid-IR) detectors. Nevertheless, their electronic structure and transport properties remain poorly understood, and some critical aspects such as the carrier relaxation dynamics at the band edge have been pushed under the rug. Some of the previous reports on dynamics are setup-limited, and all of them have been obtained using photon energy far above the band edge. These observations raise two main questions: (i) what are the carrier dynamics at the band edge and (ii) should we expect some additional effect (multiexciton generation (MEG)) as such narrow band gap materials are excited far above the band edge? To answer these questions, we developed a high-bandwidth setup that allows us to understand and compare the carrier dynamics resonantly pumped at the band edge in the mid-IR and far above the band edge. We demonstrate that fast (>50 MHz) photoresponse can be obtained even in the mid-IR and that MEG is occurring in HgTe nanocrystal arrays with a threshold around 3 times the band edge energy. Furthermore, the photoresponse can be effectively tuned in magnitude and sign using a phototransistor configuration.

Anisotropy of Structure and Optical Properties of Self-Assembled and Oriented Colloidal CdSe Nanoplatelets

Abstract Atomically flat colloidal nanoplatelets with strong one-dimensional confinement represent the most recently discovered type of quantum-confined cadmium chalcogenide nanocrystals. In almost a decade a great progress has been achieved in the colloidal synthesis of nanoplatelets and understanding of their basic optical and electronic properties. However, up until recently methods of their controlled spatial orientation were quite scarce, what in turn hindered obtaining reliable information on their anisotropic structural and optical properties arising from their shape. In this paper we provide a mini-review of recent advances in this field of study.

Modulation of inner filter effect between plasmonic Cu2−xSySe1−y and rhodamine 6G for detection of biothiols

Synthesis and engineering of nanostructures with multiple components for the acceptable biosensing platform is a hot topic research. Herein, flower-like nonstoichiometric copper chalcogenides nanocrystals (Cu2−xSySe1−y NCs) loaded with rhodamine 6G (R6G) were used as a selective and sensitive off-on fluorescent probe for biothiols based on inner filter effect (IFE). The Cu2−xSySe1−y showed the strong interband transition band in the UV–vis region and broad localized surface plasmon resonance (LSPR) in the near-infrared region which exhibited good light-harvesting features, and thus can show sufficient absorption of the excitation or emission light of fluorescent R6G with high fluorescence quenching ability. When the biothiols (glutathione, homocysteine and cysteine) were added, the interband transition bands of Cu2−xSySe1−y NCs were suppressed, resulting in the fluorescence of R6G switched on. There was a good linear relationship between fluorescent recovery of R6G and the concentrations of glutathione, homocysteine and cysteine over the range of 1.0–150 μM, 1.0–200 μM, 1.0–150 μM with a detection limit of 32.0 nM, 34.0 nM and 31.0 nM, respectively. The established strategy exhibited the selective response to biothiol compounds and was also been applied for detecting biothiols in the human serum samples.

The Effect of Dielectric Environment on Doping Efficiency in Colloidal PbSe Nanostructures.

Doping, as a central strategy to control free carrier type and concentration in semiconductor materials, suffers from low efficiency at the nanoscale, especially in systems having high permittivity (ϵ) and large Bohr radii, such as lead chalcogenide nanocrystals (NCs) and nanowires (NWs). Here, we study dielectric confinement effects on the doping efficiency of lead chalcogenides nanostructures by integrating PbSe NWs in the platform of field effect transistors (FETs). Elemental Pb or In or elemental Se is deposited by thermal evaporation to remotely n- or p-dope the NWs. Polymeric and oxide materials of varying ϵ are subsequently deposited to control the dielectric environment surrounding the NWs. Analyzing the device characteristics, we extract the change of carrier concentration introduced by tailoring the dielectric environment. The calculated doping efficiency for n-type (Pb/In) and p-type (Se) dopants increases as the ϵ of the surrounding medium increases. Using a high-ϵ material, such as HfO2 for encapsulation, the doping efficiency can be enhanced by >10-fold. A theoretical model is built to describe the doping efficiency in PbSe NWs embedded in different dielectric environments, which agrees with our experimental data for both NW array and single NW devices. As dielectric confinement affects all low-dimensional materials, engineering the dielectric environment is a promising general approach to enhance doping concentrations, without introducing excess impurities that may scatter carriers, and is suitable for various device applications.

Improved efficiency of PbS quantum dot sensitized NiO photocathodes with naphthalene diimide electron acceptor bound to the surface of the nanocrystals

Hybrid materials combining a wide bandgap metal oxide semiconductor, metal chalcogenide nanocrystals and molecular systems represent very attractive materials for fabricating devices with new function or improved photoelectrochemical performance. This study deals with sensitization of NiO, which is a p-type semiconductor, by quantum dots (QDs) of PbS with an average diameter of 3nm. The PbS QDs were attached to the monocrystalline film of NiO by mercaptopropionic acid linker and were subsequently capped with methyl-pyridine naphthalene diimide (NDI) units to prepare quantum dot sensitized solar cells (p-QDSSCs) on NiO electrodes. Time-resolved photoluminescence measurements of the PbS emission were used to determine the rate constants for charge transfer from the PbS exciton to the NiO, cobalt based redox mediator and NDI. Notably, it was shown that NDI quenches the PbS exciton by electron transfer with a quite fast rate constant (6.9 × 107s−1). The PbS QDs sensitized NiO films were finally used to fabricate solar cells with tris(4,4′-ditert-butyl-2,2′-bipyridine) cobalt(III/II) as redox mediator. It was observed that the presence of NDI on PbS improved the photovoltaic performance by 50% relative to that of cells without NDI, leading to a device with the following characteristics: Jsc = 5.75mA/cm2, Voc = 226mV, ff = 34% and PCE = 0.44%. This study demonstrates that photogalvanic processes can be a productive pathway to better performing sensitized p-type semiconductor for p-QDSSC. In other words, photoinduced electron transfer from the QDs towards the electrolyte rather than initial photoinduced charge injection into the p-type semiconductor can be a favorable operative mechanism in QD sensitized NiO films and might be exploited further for the construction of better performing solar cells or photocatalytic devices.